1. Field of the Invention
The present invention relates to a gas decompression device for fuel cell system which is used for decompressing fuel gas or oxidizing gas supplied to a fuel cell in fuel cell system.
2. Description of Related Art
JP Laid-open Patent Publication No. 2002-373682 (pp. 3–6, FIG. 1 and FIG. 2) discloses a conventional gas decompression device. Fuel cell system disclosed in No. 2002-373682 employs a regulator for reducing pressure of fuel gas (hydrogen gas) supplied to a fuel cell depending on pressure of air supplied to the fuel cell. The regulator comprises a pneumatic proportional pressure control valve wherein air supplied from an air compressor to a fuel cell is used, and a pressure signal of the supplied air is inputted so that pressure of hydrogen gas at an outlet (outlet pressure) should be reduced to a predetermined range of pressure depending on the pressure signal. Generally, this type of regulator has a diaphragm connected to a valve body and a spring such as pressure control spring and adjusts gas pressure by regulating opening of the valve body wherein resultant force of force on the diaphragm by gas and urging force of the spring is balanced.
In the conventional regulator, load of the pressure control spring changes depending on displacement of the valve body (displacement of the diaphragm). As a result, outlet pressure changes accordingly. That is, when the diaphragm is displaced, the spring is transformed and urging force of the spring is changed in accordance with a spring constant. As shown in FIG. 9, direction of action is opposite to each other between gas pressure on the diaphragm and urging force of the pressure control spring, however, those forces decline as gas flow rate (opening of the valve body) increases. Therefore, as shown in FIG. 10, outlet pressure declines as gas flow rate (opening of the valve body) increases. Primarily, it is desirable that outlet pressure of a regulator for fuel cell system be constant irrespectively of large/small of gas flow rate. However, the conventional regulator cannot evade influence of a spring constant, and its outlet pressure declines in case large amount of hydrogen gas flows into the regulator.
Furthermore, in the conventional fuel cell system, pressure of air supplied from the air compressor to the regulator changes following up flow rate of air supplied from the air compressor to the fuel cell. Therefore, air pressure needed for the regulator cannot be supplied to the regulator separately. As a result, it is difficult to adjust outlet pressure of hydrogen gas in the regulator as desired.
Furthermore, a stop of the fuel cell system makes the valve body of the regulator enclosed completely, whereby, as shown in FIG. 10, outlet pressure obtained when gas flow rate is “0” becomes high. Therefore, there has been required pressure proof design to thicken devices to be arranged at the downstream side of the regulator. Such pressure proof design makes the entirety of a device grow in size and period of endurance tends to be shortened.